1. Field of the Invention
The present invention relates to a lithographic apparatus having a controlled motor, and a motor control system and method.
2. Description of the Related Art
A lithographic apparatus is a machine that applies a desired pattern onto a substrate, usually onto a target portion of the substrate. A lithographic apparatus can be used, for example, in the manufacture of integrated circuits (ICs). In such a case, a patterning device, which is alternatively referred to as a mask or a reticle, may be used to generate a circuit pattern to be formed on an individual layer of the IC. This pattern can be transferred onto a target portion (e.g. including part of, one, or several dies) on a substrate (e.g. a silicon wafer). Transfer of the pattern is typically via imaging onto a layer of radiation-sensitive material (resist) provided on the substrate. In general, a single substrate will contain a network of adjacent target portions that are successively patterned. Conventional lithographic apparatus include so-called steppers, in which each target portion is irradiated by exposing an entire pattern onto the target portion at once, and so-called scanners, in which each target portion is irradiated by scanning the pattern through a radiation beam in a given direction (the “scanning”-direction) while synchronously scanning the substrate parallel or anti-parallel to this direction. It is also possible to transfer the pattern from the patterning device to the substrate by imprinting the pattern onto the substrate.
A lithographic apparatus includes a plurality of movable objects which are to be accurately positioned with six degrees of freedom (in an XYZ coordinate system: x, y, z, Rx, Ry, Rz). Examples of such movable objects are a substrate stage and a reticle stage. The objects may be driven and positioned by controlled planar electric motors each comprising an essentially planar stator and a mover, where the mover is the part of the motor being generally translatable relative to the stator. The mover generally moves in a direction parallel to a plane of the stator, with a gap being present between the stator and the mover.
Planar motors may comprise permanent magnets in either the stator or the mover for generating a magnetic field in which current conducting coils in the other part, either the mover or the stator, generate a force for moving the stator and mover relative to each other. A planar motor of a known design has a back plate or similar structure, made from a magnetizable material, with a checkerboard pattern of alternating polarized permanent magnets mounted on the backplate as a first part, and a set of coils as a second part, where the first part and the second part are movable relative to each other. In the motor, a force parallel to an x-y plane in which the first part extends (hereinafter also indicated as a horizontal force), and the forces at right angles (z-direction) thereto (hereinafter also indicated as a vertical force), are generated by the currents in the coils, where a relation between the currents and the forces is an x and y position dependent function of the first part relative to the second part according to Equations [1]:
                                          F            H                    =                      i            ·                          3              2                        ·            Km            ·                          cos              ⁡                              (                ϕ                )                                                    ⁢                                  ⁢        and        ⁢                                  ⁢                              F            V                    =                      i            ·                          3              2                        ·            Km            ·                          sin              ⁡                              (                ϕ                )                                                                        [        1        ]                            wherein:        FH horizontal force        FV vertical force        i current        Km motor constant        φ commutation angle        
In the Equations [1], a motor constant Km appears which is, inter alia, determined by the coils' geometry and by the magnitude of the magnetic field generated by the permanent magnets of the planar motor.
A planar motor used in a lithographic apparatus, for example a planar long stroke motor used to drive a substrate stage, is designed to generate high acceleration forces, and to reach high velocities to maximize the throughput of the lithographic apparatus. This requires high currents to flow in the coils of such a motor, and the power dissipation in the coils of the motor consequently is high. During operation, a surface of the coils may heat up several tens of degrees. Primarily via heat conduction through a medium present in the gap between the coils and magnets, such as air, the magnets (which are facing the coils) may heat up several degrees when the coils heat up, depending on the way of cooling the coils and/or the magnets.
The magnetic field generated by the permanent magnets has a temperature sensitivity, and, as a consequence, a temperature change of the permanent magnets will lead to a change of the magnetic field, and thus a variation of the motor constant of the planar motor concerned. As an example, a temperature sensitivity of the permanent magnets may be in the order of e.g. −0.3%/K, where a temperature rise of several kelvin will lead to a loss of magnetic field of several percent. The temperature sensitivity thus may lead to appreciable errors in a control of the motor position, in particular of the feedforward control of the motor position.